You can calculate the “Fair Market Value” of the stock(s) you donated by averaging the highest and lowest price of that stock on the day you donated it. I used this page to find that, but you can replace “MSFT” in the URL with whatever stock it is you sold.
Update for anyone else who may find it useful:
You need to fill out Form 8283: https://www.irs.gov/pub/irs-pdf/f8283.pdf
You can calculate the “Fair Market Value” of the stock(s) you donated by averaging the highest and lowest price of that stock on the day you donated it. I used this page to find that, but you can replace “MSFT” in the URL with whatever stock it is you sold.
https://www.wsj.com/market-data/quotes/MSFT/historical-prices